Field of the Disclosure
The present invention relates generally to chemically prepared toner having a core shell structure for use in electrophotography and more particularly to emulsion aggregation chemically prepared toner including a borax coupling agent located on the surface of the core and a plasticizing agent located within the core of the toner.
Description of the Related Art
Toners for use in electrophotographic printers include two primary types, mechanically milled toners and chemically prepared toners (CPT). Chemically prepared toners have significant advantages over mechanically milled toners including better print quality, higher toner transfer efficiency and lower torque properties for various components of the electrophotographic printer such as a developer roller, a fuser belt and a charge roller. The particle size distribution of CPTs is typically narrower than the particle size distribution of mechanically milled toners. The size and shape of CPTs are also easier to control than mechanically milled toners.
There are several known types of CPT including suspension polymerization toner (SPT), emulsion aggregation toner (EAT)/latex aggregation toner (LAT), toner made from a dispersion of pre-formed polymer in solvent (DPPT) and “chemically milled” toner. While emulsion aggregation toner requires a more complex process than other CPTs, the resulting toner has a relatively narrower size distribution. Emulsion aggregation toners can also be manufactured with a smaller particle size allowing improved print resolution. The emulsion aggregation process also permits better control of the shape and structure of the toner particles which then allows the toner particles to be tailored to fit the desired cleaning, doctoring and transfer properties. The shape of the toner particles produced from an EA process may be optimized to ensure proper and efficient cleaning of the toner from various electrophotographic printer components, such as the developer roller, charge roller and doctoring blades, in order to prevent filming or unwanted deposition of toner on these components.
In a typical process for preparing EAT, emulsion aggregation is carried out in an aqueous system resulting in good control of both the size and shape of the toner particles. The toner components typically include a polymer binder, one or more colorants and a release agent. A styrene-acrylic copolymer polymer binder is often used as the latex binder in the emulsion aggregation process. However, the use of a styrene-acrylic copolymer latex binder requires a tradeoff between the toner's fusing properties and its shipping and storage properties. One important characteristic of any toner is its fuse window. The fuse window is the range of temperatures at which fusing is satisfactorily conducted without incomplete fusion and without transfer of toner to the heating element, which may be a roller, belt or other member contacting the toner during fusing. Thus, below the low end of the fuse window the toner is incompletely melted and above the high end of the fuse window the toner flows onto the fixing member where it mars subsequent sheets being fixed. It is preferred that the low end of the fuse window be as low as possible to reduce the required temperature of the fuser in the electrophotographic printer to therefore improve the printer's safety and to conserve energy.
However in addition to fuse at an energy saving low temperature, the toner must also be able to survive the temperature and humidity extremes associated with storage and shipping—commonly called the ship/storage test. Caking or blocking of the toner during shipping and storage usually results in print flaws. Energy saving low fusing toner is desirable but the low end of the fuse window cannot be so low that the toner melts during the storing or shipping of a toner cartridge containing the toner. A low melt/low energy fusing toner must be robust to shipping and storage conditions in order to be attractive in a worldwide market. However, many toner formulations cannot simultaneously meet the demand to fuse at low temperatures while also passing the ship/storage tests.
Toners formed from polyester binder resins typically possess better mechanical properties than toners formed from a styrene-acrylic copolymer binder of similar melt viscosity characteristics. This makes them more durable and resistant to filming of printer components. Polyester toners also have better compatibility with color pigments resulting in a wider color gamut. Until recently, polyester binder resins were frequently used in preparing mechanically milled toners but rarely in chemically prepared toners. Polyester binder resins are manufactured using condensation polymerization. This method is time consuming due to the involvement of long polymerization cycles and therefore limits the use of polyester binder resins to polyester polymers having low to moderate molecular weights, which limits the fusing properties of the toner. Further, polyester binder resins are more difficult to disperse in an aqueous system due to their polar nature, pH sensitivity and gel content thereby limiting their applicability in the emulsion aggregation process.
However with advancement in toner manufacturing technology, many toner manufacturers are now using polyester resins rather than styrene acrylic resins because it has become possible to obtain stable polyester emulsions. These stable emulsions are formed using polyester binder resins by first dissolving them in an organic solvent, such as methyl ethyl ketone (MEK), methylene chloride, ethyl acetate, or tetrahydrofuran (THF), and then performing a phase-inversion process where water is added slowly to the organic solvent. The organic solvent is then evaporated to allow the polyester binder resins to form stable emulsions. U.S. Pat. No. 7,939,236 entitled “Chemically Prepared Toner and Process Therefor,” which is assigned to the assignee of the present application and incorporated by reference herein in its entirety teaches a similar process for obtaining a stable polyester emulsion using an organic solvent. These advances in producing stable polyester emulsions have permitted the increased use of polyester binder resins to form emulsion aggregation toner. For example, U.S. Pat. No. 7,923,191 entitled “Polyester Resin Produced by Emulsion Aggregation” and U.S. patent application Ser. No. 12/206,402 entitled “Emulsion Aggregation Toner Formulation,” which are assigned to the assignee of the present application and incorporated by reference herein in their entirety, disclose processes for preparing emulsion aggregation toner using polyester binder resins.
However, simple switching from a styrene-acrylate resin to a to polyester resin in an EA toner formulation does not completely meet the challenges of providing a toner formulation that is energy efficient and survive shipping and storage concerns while providing great print quality. Unfortunately, toners having low molecular weight polyester resins do not significantly open the low temperature end of the fuse window to allow the toner to be energy efficient. Moreover due to its short chain migration speed, the amount of the polyester resin must be limited in the toner formulation in order for the toner to survive the temperatures and humidity extremes when being shipped and stored. The inventors of the present invention believe that lower fusing temperatures in a toner can be achieved by the addition of plasticizing agents into the toner formulation. Any ideal plasticizing agent must possess not only low melting temperatures, but also have a sufficiently low melt flow viscosity to enable the toner to penetrate into paper fibers thereby giving the toner good fixation under such low melting temperatures. Also the plasticizing agent must provide enough filming strength to withstand the lifting/peeling actions at higher printing speeds at the operational temperature range of the electrophotographic printer. Unfortunately, most of the known plasticizing agents are low molecular weight organic compounds or polymers and tend to migrate to the surface of the toner during the manufacture of the toner which negatively impacts the toner's ship/store properties.
Accordingly, it will be appreciated that a toner formulation and process that can simultaneously fuse at an energy saving low temperature in addition to survive shipping and storage concerns while providing good print quality is desired. It is also desired to minimize the overall number of fine toner particles, which contribute to filming on the printer components.